1. Field of the Invention
This invention relates to voltage regulators and, more specifically, to pulse width modulated switching voltage regulators. It is particularly concerned with a switching voltage regulator having digital feedback control utilizing frequency averaging techniques.
2. Prior Art
The conventional switching-type voltage regulator utilizes analog-type signal circuitry. The conventional switching regulator generally includes a switch, a switching drive, and a filter circuit as its main components. The switch is operated by the switching drive to periodically chop an applied DC input voltage. The chopped voltage is applied to the filter which derives therefrom an average DC voltage.
The switch is usually a transistor or SCR device which is operated in a fully conducting state, and an alternately nonconducting state. The drive circuit for the switch is generally coupled with a feedback circuit to regulate the output voltage. In the pulse width modulated type of switching regulator the pulse duty cycle is varied as a function of a line or load signal magnitude to compensate for changes therein. In most instances a regulated output voltage is compared with a reference voltage and the duty cycle of the switching device is modified to maintain the output voltage at a desired regulated value.
Analog feedback circuitry, while simple to implement, is difficult to conrol with respect to signal variations due to environmental factors. Such factors include the ambient temperature in which the circuit operates, the age of the circuitry, and other characteristic variations in the individual components of the circuit. Various compensating schemes have been devised to combat these disadvantageous environmental factors. Such schemes include component compensation circuits which add to the complexity of the circuitry. Other schemes require a careful coordinated selection of circuit components whose changes in response to environmental factors tend to cancel each other.
Recent switching regulator designs are beginning to utilize digital components in the regulation circuitry. An example of a digital feedback scheme differing from the conventional analog scheme is disclosed in U.S. Pat. No. 3,445,754, issued to S. L. Broadhead on May 20, 1969. In this switching type voltage regulator the conduction through a switching device is pulse width modulated in response to the comparison of the signal of a reference oscillator and the signal of a voltage controlled oscillator responsive to the regulated output voltage. The input DC voltage is coupled to a switching device. The conductivity of the switching device is controlled by a flip-flop circuit whose RS inputs are triggered respectively by the reference oscillator output and the voltage controlled oscillator output. The phase relation between the voltage controlled oscillator and the reference oscillator controls the duration of the DC signal voltage transmitted by the switching device to the output of the voltage regulator. This pulse width modulation arrangement controls the magnitude of the output voltage.
The Broadhead feedback control circuit is a phase-locked loop oscillator control for a voltage regulator which includes a voltage controlled oscillator. This control uses the signal phase difference between the voltage controlled oscillator output and the reference oscillator output to control the output of the regulator. While the form of the reference signal is different from the form of the output signal, this is still an analog signal approach and includes many of the same disadvantages due to environmental factors.
It is specifically an object of the present invention to control the duty cycle of a switching regulator with digital techniques exclusive of analog techniques.
It is particularly an object of the invention to replace the analog feedback control of the conventional switching regulator with a digital feedback control.
It is more particularly an object to establish a reference signal which is digital and which is digitally manipulated to improve the regulation response of the switching regultor.
It is a further object to establish an invariant reference signal which is not affected by environmental factors.